Stents are typically used as adjuncts to percutaneous transluminal balloon angioplasty procedures, in the treatment of occluded or partially occluded arteries and other blood vessels. As an example of a balloon angioplasty procedure, a guiding catheter or sheath is percutaneously introduced into the cardiovascular system of a patient through a femoral artery and advanced through the vasculature until the distal end of the guiding catheter is positioned at a point proximal to the lesion site. A guidewire and a dilatation catheter having a balloon on the distal end are introduced through the guiding catheter with the guidewire sliding within the dilatation catheter. The guidewire is first advanced out of the guiding catheter into the patient's vasculature and is directed across the vascular lesion. The dilatation catheter is subsequently advanced over the previously advanced guidewire until the dilatation balloon is properly positioned across the vascular lesion. Once in position across the lesion, the expandable balloon is inflated to a predetermined size with a radiopaque liquid at relatively high pressure to radially compress the atherosclerotic plaque of the lesion against the inside of the artery wall and thereby dilate the lumen of the artery. The balloon is then deflated to a small profile so that the dilatation catheter can be withdrawn from the patient's vasculature and blood flow resumed through the dilated artery.
Balloon angioplasty sometimes results in short or long term failure. That is, vessels may abruptly close shortly after the procedure or restenosis may occur gradually over a period of months thereafter. To counter restenosis following angioplasty, implantable intraluminal prostheses, commonly referred to as stents, are used to achieve long term vessel patency. A stent functions as scaffolding to structurally support the vessel wall and thereby maintain luminal patency, and are transported to a lesion site by means of a delivery catheter.
Types of stents may include balloon expandable stents, spring-like, self-expandable stents, and thermally expandable stents. Balloon expandable stents are delivered by a dilitation catheter and are plastically deformed by an expandable member, such as an inflation balloon, from a small initial diameter to a larger expanded diameter. Self-expanding stents are formed as spring elements which are radially compressible about a delivery catheter. A compressed self-expanding stent is typically held in the compressed state by a delivery sheath. Upon delivery to a lesion site, the delivery sheath is retracted allowing the stent to expand. Thermally expandable stents are formed from shape memory alloys which have the ability to expand from a small initial diameter to a second larger diameter upon the application of heat to the alloy.
It may be desirable to provide localized pharmacological treatment of a vessel at the site being supported by a stent. Thus, sometimes it is desirable to utilize a stent both as a support for a lumen wall as well as a delivery vehicle for one or more pharmacological agents. Unfortunately, the bare metallic materials typically employed in conventional stents are not generally capable of carrying and releasing pharmacological agents. Previously devised solutions to this dilemma have been to join drug-carrying polymers to metallic stents. Additionally, methods have been disclosed wherein the metallic structure of a stent has been formed or treated so as to create a porous surface that enhances the ability to retain applied pharmacological agents. However, these methods have generally failed to provide a quick, easy and inexpensive way of loading drugs onto intraluminal prostheses, such as stents. In addition, only small amounts of drugs can be loaded into thin polymeric coatings.
Intraluminal prostheses, such as stents have been developed using various polymeric materials and/or coatings of polymeric materials to overcome the limitations of conventional metallic prostheses. However, it would be desirable to be able to adjust various mechanical properties (e.g., modulus, hoop strength, flexibility, etc.) of polymeric intraluminal prostheses. For example, for intraluminal prostheses used to deliver pharmacological agents, it would be desirable to be able to adjust the elution rate of a pharmacological agent therefrom. As another example, it would be desirable to be able to adjust the degradation rate and/or the nature of degradation of the polymeric material.